Systems And Methods Utilizing Randomized Clock Rates To Reduce Systematic Time-Stamp Granularity Errors In Network Packet Communications

ABSTRACT

Systems and methods are disclosed for utilizing slave (receive) time-stamp clock rates that are different from master (sender) time-stamp clock rates to randomize and thereby reduce systematic time-stamp granularity errors in the communication of network packets. The slave (receive) time-stamp clock rate for some embodiments is set to be a fixed value that has a relationship with the master (sender) time-stamp clock rate such that the ratio of the slave (receive) clock rate to the master (sender) clock rate is a rational number. Other embodiments use a time-varying frequency for the slave (receive) time-stamp clock rate to randomize the slave (receive) time-stamp clock with respect to the master (sender) time-stamp clock. Additional time-stamps can also be generated using a slave (receive) time-stamp clock having a rate set to equal the rate of the master (sender) time-stamp clock signal. Further spread spectrum and/or delta-sigma modulation techniques can be applied to effectively randomize the slave (receive) time-stamp clock.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/442,262, filed Apr. 9, 2012, and entitled “SYSTEMS ANDMETHODS UTILIZING RANDOMIZED CLOCK RATES TO REDUCE SYSTEMATIC TIME-STAMPGRANULARITY ERRORS IN NETWORK PACKET COMMUNICATIONS,” which claimspriority to U.S. Provisional Patent Application Ser. No. 61/539,806,filed Sep. 27, 2011, and entitled “SYSTEMS AND METHODS UTILIZINGRANDOMIZED SLAVE TIME-STAMP CLOCK RATES TO REDUCE SYSTEMATIC TIME-STAMPGRANULARITY ERRORS IN NETWORK PACKET COMMUNICATIONS,” each of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the use of time-stamps in network packetcommunications and, more particularly, to time-stamp granularity errorsin network communications.

BACKGROUND

One of the more important requirements of a digital communicationnetwork is to support real-time communications applications which,typically, require time or frequency alignment, or a combination ofboth. For example, time alignment is used by real-time instrumentationsystems gathering data at specific time intervals or operating machineryaccording to specific timing. Frequency alignment is required intime-division-multiplexed (TDM) systems and in multi-media streamingsystems, which require fixed, reproducible, video or audio sample ratesacross multiple clients.

While frequency alignment within conventional TDM networks is relativelystraightforward, packet-switched networks, such as networks based onInternet Protocols (“IP”), present time and frequency alignmentchallenges because packet networks are not conventionally designed toprovide precise delivery time for data or precise timing. A keydifference is that the switching and multiplexing functions are notdeterministic, as they are in TDM networks, but have a random aspect aswell. In particular, packet networks typically involve multiple nodesthat may store and forward data packets, potentially introducingsignificant, randomly distributed, transit delay variation between anytwo points.

To address time and frequency alignment challenges inherent in packetnetworks, certain protocols based on the industry standard internetprotocol (IP) have been developed and deployed. One IP-based timealignment protocol is known as the Network Time Protocol (NTP). NTP isused for aligning time between a client and one or more master timereferences. Precision Time Protocol (PTP) is a second IP-based timealignment protocol for aligning one or more client devices to a mastertime reference. Both NTP and PTP provide for the use of syntonizedtime-stamp clocks (i.e, they are aligned in frequency) in master(sending) devices and slave (receiving) devices to produce time-stampsfor achieving this alignment of time references. Master devices can beany of a variety of devices (e.g., communication sources, servers, etc.)that are sending packets to slave devices, which in turn can be any of avariety of devices that are receiving packets (e.g., communicationdestinations, clients, etc.).

FIG. 1 (Prior Art) is a conceptual deployment diagram that illustrates aslave clock in a slave device (destination/slave/client) 102synchronizing to a master clock in a master device(source/master/server) 101 over a packet network 103. The packet timingsignals 104 communicated over the packet network 103 includetime-stamped packets exchanged between the master and slave devices at anominally regular interval with, for example, T₀ time units (e.g.,seconds) between packets. In FIG. 1 (Prior Art), the transit delay fromthe master device (source/master/server) 101 to the slave device(destination/slave/client) 102 is represented by the Δ_(MS) arrow 108.And the transit delay from the slave device (destination/slave/client)102 to the master device (source/master/server) 101 is represented bythe Δ_(SM) arrow 109. The slave device (destination/slave/client) 102uses time-stamps associated with the packet timing signals 104 togenerate local clock signals represented by clock output 105.

The nature of the timing signals is provided in FIG. 2 (Prior Art). Thetiming packet that traverses the network from master to slave leaves themaster 210 at time t₁ (labeled as “A”). This constitutes thetime-of-departure time-stamp. After a transit delay of Δ_(MS) 240, thepacket arrives at the slave 220 at time t₂ (labeled as “B”). The slavemeasures this time-of-arrival as τ₂ based on the slave clock. In otherwords, the time-stamp for time-of-arrival has the value τ₂. Denoting byε the time offset of the slave clock relative to the master clock, thetime t₂ for the time-of-arrival at the slave 220 can be represented bythe following equation:

t ₂=τ₂+ε  (Eq. 1)

For packets originating at the slave 220 and transmitted to the master210, the time-of-departure time-stamp contains τ₃, which represents theslave-clock estimate of time-of-departure. The actual (based on themaster timescale) time-of-departure t₃ (labeled as “C”) is related to τ₃by the following equation:

t ₃=τ₃+ε  (Eq. 2)

The transit delay across the network is Δ_(SM) 244. Thus, after atransit delay of Δ_(SM) 244, the packet arrives at the master 210 attime t₄ (labeled as “D”).

Such a two-way exchange of packets can provide information suitable forallowing the slave to align in time with the master (assuming that bothsides have knowledge of the time stamps). There are four measured values(time-stamps) that can be communicated between the Master and Slave,namely, timestamps associated with t₁, τ₂, τ₃, and t₄, which aredescribed with respect to FIG. 2 (Prior Art). It is noted that thistwo-way exchange involves one packet (message) in each direction, andthese messages do not necessarily have to be consecutive as long as thetime-stamp information is communicated appropriately. In some instances,the rate at which packets are transmitted in the two directions can bedifferent.

Denoting as Δ_(MS) and Δ_(SM) the transit delays between theMaster-to-Slave communication and Slave-to-Master communication,respectively, the following equations can be established:

t ₄=τ₃+ε+Δ_(SM)(from a S-to-M packet)

t ₁=τ₂+ε−Δ_(MS)(from a M-to-S packet)  (Eq. 3)

In this actual time-transfer situation, there are two equations withthree unknowns. As such, it is common practice to assume reciprocity oftransit delay between the two devices (i.e., assuming that Δ_(MS) equalsΔ_(SM)), thereby reducing the number of unknowns to two. Making thisassumption and computing for ε, this slave wall-clock timeoffset-from-master (“ofm”) value ε can be indicated as follows:

$\begin{matrix}{ɛ = {\frac{\left( {t_{4} + t_{1}} \right) - \left( {\tau_{3} + \tau_{2}} \right)}{2} = \frac{\left( {t_{4} - \tau_{3}} \right) - \left( {\tau_{2} - t_{1}} \right)}{2}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

There are several phenomena that interfere with the accuracy andprecision whereby the slave clock can determine its timeoffset-from-master. One such error contribution is related to the basicnature of packet-switched networks whereby the transit delay of packetsfrom source to destination can vary from packet to packet. This effect,also called packet delay variation (PDV), restricts the ability of theslave to align with the master perfectly. Further, because theoscillator in the slave clock is not ideal, it could drift betweenpacket transmissions, and the assumption that the slave time clockoffset-from-master (ε) is “constant” over the interval of packetexchange is not exact. Several methods and techniques have beendisclosed to address these kinds of error sources and mitigate theirimpact on the performance of the clock. These methods commonly applylow-pass filtering. That is, the noise type is random, and by applyingsuitable filtering methods, the noise power is reduced, improving thesignal-to-noise ratio.

It is noted that for very high precision/accuracy time-transfer inpacket networks, there are no intervening devices between the Master andthe Slave. In PTP architectures, this lack of intervening devices isreferred to as the networking providing “on-path support,” and eachintervening device between the end-point clocks is called a “boundaryclock.” In effect, each boundary clock is a slave clock on one side(e.g., the “upstream” or “Master” side) and a master clock on the otherside (e.g., the “downstream” or “Slave” side). In this situation, thetransit delay of packets from master/source to slave/destination isnominally constant.

One other source of error that has not been addressed previously is theerror in the time-stamp value itself. If the time-stamp functionprovides an erroneous value for the time-of-departure or thetime-of-arrival of a packet, then this error propagates into theestimate of the slave clock time offset-from-master. As described belowwith respect to FIG. 3A (Prior Art) and FIG. 3B (Prior Art), priormethods for generating time-stamps can introduce a time-stamp error thatis static and, therefore, cannot be filtered out by traditional means.In boundary clock scenarios, this source of error is one of the primarycontributors to the error in typical time transfer systems.

FIG. 3A (Prior Art) is a block diagram of an embodiment 300 forcircuitry that generates a time-stamp for network packets. For thisembodiment 300, a nominal packet rate is considered to be f₀, and thenominal packet interval is considered to be T₀, such that f₀=1/T₀. Asshown in FIG. 3A (Prior Art), the time-stamp clock 306 (e.g., clockrate=f_(TS)) runs a counter 302. Transitions in the least-significantbit are associated with a time-equivalency of ΔTS=1/f_(TS). (In someimplementations, the increment is other than unity, allowing a differentweight (in time units) than Δ_(TS)=1/f_(TS).) Thus, the effectivegranularity of the time-stamp is Δ_(TS).

For the embodiment 300 depicted, the event to be time-stamped (e.g.,receipt of a packet from the network for time-of-arrival) generates aload-pulse 308 based upon an event trigger. This load-pulse 308 causesthe counter 302 to be sampled to generate a time-stamp value, and thistime-stamp value 303 is stored in the time-stamp register 304, which canoutput a time-stamp of the last event, if desired, or can be accessed insome other desired manner The event time-stamps 305 can then be receivedand used in a slave device (Slave), for example, in synchronizing itsoperations to a clock from a master device (Master) on the network.

It is noted that it is incorrect to assume that the time-of-arrival of apacket is necessarily aligned to an edge of the time-stamp clock(f_(TS)). Consider the case of time-stamp T₂ representing the Slave'sestimate of the time-of-arrival of a packet sent by the Master, asdiscussed in more detail with respect to FIG. 3B (Prior Art) below. TheSlave's time-stamp mechanism does not influence what time the Mastersends the packet, and also the Slave does not have any control over thetransit delay of the packet over the medium. In the Slave, the signalprocessing, for example, associated with demodulation of the signal,clock-and-data recovery, and/or determination of the appropriate bitrate, initiates a strike of a slave time-stamp based upon an event, suchas the load pulse 308 in FIG. 3A (Prior Art). The Slave time-stamp clockedge used to generate the time-stamp is likely not to align exactly withthe actual time of receipt of the packet.

Thus, all events that occur between nΔ_(TS) and (n+1)Δ_(TS) will mapinto the same time-stamp value for the Slave device. This ability togenerate time-stamps for events only at the precision of the time-stampclock is the notion of quantization or granularity of the time-stamp.Put another way, there is an uncertainty of Δ_(TS) associated with atime-stamp as compared to the actual time of the event in continuoustime.

In practice, time-stamps are taken in pairs. Specifically, the timeoffset-from-master (ofm) as computed by the Slave is given by thefollowing equation:

$\begin{matrix}{{{ofm}(n)} = \frac{\left( {{T_{4}(n)} + {T_{1}(n)}} \right) - \left( {{T_{3}(n)} + {T_{2}(n)}} \right)}{2}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

where T₄(n) and T₁(n) are the time-of-arrival and time-of-departuretime-stamps struck at the Master and T₂(n) and T₃(n) are thetime-of-arrival and time-of-departure time-stamps struck at the Slave.The index (n) represents the notion of the ofm being calculated usingthe n^(th) packet exchange.

The primary reason for associating the time-stamps as shown in Equation5 above is to recognize that T₁ and T₄ are “Master” time-stamps(generically designated in the figures as TS_(M)) and T₂ and T₃ are“Slave” time-stamps (generically designated in the figures as TS_(S)).Consequently T₁ and T₄ are struck at the same port and, likewise, T₂ andT₃ are struck at the same port. On the slave side, the time-stampgranularity noise can be written as

ε_(TS) ^((S))(n)=−½(ε₃(n)+ε₂(n))  (Eq. 6)

where ε₃(n) and ε₂(n) are the time-stamp granularity noise contributionsfor time-stamps T₃(n) and T₂(n), respectively. The negative sign issolely for convenience. When considering errors, the more relevantmetrics are related to power, and for power, the sign is moot. Likewise,on the Master side, the time-stamp granularity noise can be written as

ε_(TS) ^((M))(n)=½(ε₄(n)+ε₁(n))  (Eq. 7)

where ε₄(n) and ε₁(n) are the time-stamp granularity noise contributionsfor time-stamps T₄(n) and T₁(n), respectively.

FIG. 3B (Prior Art) is a timing diagram of an embodiment 350 forcommunication of a packet involving a Master time-stamp 356 and a Slavetime-stamp 358. For prior solutions, the clock rate for the Mastertime-stamp clock 354 is the same as the clock rate for the Slavetime-stamp clock 352. The time-of-departure 360 represents when thepacket is transmitted by the Master and first time-stamp (T₁) 356represents the Master time-stamp. The time-of-arrival 364 representswhen the packet is received by the Slave, and the flight time 362represents the time the packet takes to travel from Master to Slave. Thesecond time-stamp (T₂) 358 represents the time-stamp struck at the Slavefor the transmitted packet. The difference between the clock edgerepresenting this time-stamp (T₂) 358 and the actual time-of-arrival 364is represented as time difference (Δ) 370.

Considering this case where a time-stamped packet is transmitted fromthe Master to the Slave, the time-of-departure 360 is assumed to bealigned with an edge of the time-stamp clock in the master, although itcould be misaligned in practice. However, this misalignment is static innature. The packet is delivered over the medium, for example, at 1Gbit/s Ethernet, and arrives at the Slave (receiver) where thetime-of-arrival is established. For a given situation, the flight time362 over the medium can be considered essentially constant from packetto packet.

In legacy and conventional network communication systems, the time-stampclock 352 in the Slave (receiving device) is implemented using the samenominal rate as that of the time-stamp clock 354 for the Master (sendingdevice). A typical value for the clock rate for 1 Gbit/s Ethernet, forexample, is 125 MHz, corresponding to a time-stamp time granularity of 8ns.

As depicted in FIG. 3B (Prior Art) a time-stamped packet leaves theMaster (time-stamp=T₁) and it leaves on the rising edge of thetime-stamp clock 354. The time-stamp 356 has no granularity errorassociated with it because of the assumption that the time-of-departure360 is aligned with an edge of the time-stamp clock 354.

The packet arrives at the Slave (receiver) after some time representedby the flight time 362. At the Slave (receiver), there is somegranularity error because there is no guarantee that the time-of-arrival364 will be in alignment with an edge of the time-stamp clock 352. Asdepicted in embodiment 350, the time-stamp associated withtime-of-arrival 364 will be the value of the clock at the closest edgeprior to the time-of-arrival. This is designated as slave time-stamp(T₂) 358. The time-stamp granularity error is Δ or the difference 370between the clock edge for time-stamp clock 352 and the actual time ofarrival 364.

Three cases can be considered for different aspects of errors associatedwith the time-stamp by the Slave (receiver).

First is a case where the slave clock 352 is syntonized with the masterclock 356 (i.e., they are aligned in frequency). In this case, the ratesof the time-stamp clocks nominally will be the same, and as aconsequence, the phase difference between the two clock waveforms shownin FIG. 3B (Prior Art) will be a constant if the two clocks have thesame nominal rate (in Hz). Because the flight time 362 can be assumed tobe the same for each packet, it follows that the time-stamp granularityerror (Δ) 370 will be the same for each packet. Such a systematicgranularity error is impossible to filter out in subsequent processing.This effect is sometimes referred to as the “beating effect.” The slavetime-clock will thus be different from the master time-clock because ofthis systematic phenomenon, and the error will be some value between 0and Δ_(TS) where Δ_(TS) is the period of the time-stamp clock (e.g., 0-8ns for a 125 MHz clock).

Next is a case where the slave clock is almost syntonized with themaster clock (i.e., they are almost aligned in frequency and the offsetis small and almost constant and the nominal rate of the time-stampingclocks is the same). In this case, the frequency, and therefore therate, of the time-stamp clocks 352 and 354 will be the almost same. As aconsequence, the phase difference between the two clock waveforms willbe almost the same and will change slowly over time. Because the flighttime 362 is again assumed to be the same for each packet, it followsthat the time-stamp granularity error (Δ) 370 will almost be the samefor each packet. Such a low-frequency error is hard to filter out, forexample, using a low-pass filter. The slave time-clock will thus bedifferent from the master time-clock because of this phenomenon, and theerror will be some value between 0 and Δ_(TS) where Δ_(TS) is the periodof the time-stamp clock (e.g., 0-8 ns for a 125 MHz clock), and thiserror will change slowly over time.

Third is a case where the slave clock is not syntonized with the masterclock (i.e., they are not aligned in frequency). For this case, even ifthe nominal rate of the time-stamp clocks is the same, they will driftwith respect to each other. As a consequence the phase differencebetween the two clock waveforms shown in FIG. 3B (Prior Art) will not bea constant. Because the flight time 362 is the same for each packet, itfollows that the time-stamp granularity error (Δ) 370 will not be thesame for each packet and will be have high Fourier frequency content.Such a high frequency error is possible to filter out, for example,using low-pass filter techniques. However, this situation will likelyoccur only during start-up and reset of the Slave/Master devices. Insteady-state operation, the Slave is synchronized to the Master, andthis implies that the slave clock and master clock will almost be thesame frequency, and this operation will therefore reduce to either thefirst or second case described above.

These cases will occur, for example, between Master and Slavecommunications for Gigabit/sec Ethernet communications where the MII(media independent interface) clock rate (e.g., 125 MHz) is used by boththe Master and Slave devices to generate time-stamps for networksynchronization.

Prior systems and techniques do not provide the ability to handle thesecases where small systematic granularity errors exist with respect toMaster and Slave time-stamp clocks that are operating at the samenominal rate, such as the MII (media independent interface) clock rate(e.g., 125 MHz) for Gigabit/sec Ethernet communications.

SUMMARY OF THE INVENTION

Systems and methods are disclosed for utilizing slave (receive)time-stamp clock rates that are different from the master (sender)time-stamp clock rates to randomize and thereby reduce systematictime-stamp granularity errors in the communication of network packets.In some embodiments, the slave (receive) time-stamp clock rate is set tobe a fixed value that has a relationship with respect to the master(sender) time-stamp clock rate such that the ratio of the slave(receive) clock rate to the master (sender) clock rate is a rationalnumber. Further, this rational number preferably has a large denominatorto represent that many cycles of the master (sender) time-stamp clockare required before its edge aligns with an edge of the slave (receive)time-stamp clock. Other embodiments use a time-varying frequency for theslave (receive) time-stamp clock rate to randomize the slave (receive)time-stamp clock with respect to the master (sender) time-stamp clock.This time-varying frequency can also be implemented so that over time,the average value for the slave (receive) time-stamp clock rate is equalto a rational multiple of an average value for the master (sender)time-stamp clock rate, although this time-varying approach is alsoapplicable for the case where the master (sender) and slave (receive)time-stamp clock rates are different over time. In further embodiments,spread spectrum techniques (e.g., applying a pseudo-random spreadingcode) or delta-sigma modulation techniques are applied to effectivelyrandomize the slave (receive) time-stamp clock to achieve thistime-varying result. Still further, additional time-stamps can begenerated using a traditional slave (receive) time-stamp clock signalhaving a rate set to equal the rate of the master (sender) time-stampclock signal. These additional time-stamps can be used along with therandomized time-stamps described herein and the master (sender)time-stamps to identify the existence of a systematic granularity errorand its magnitude. Other features and variations can be implemented, ifdesired, and related systems and methods can be utilized, as well.

DESCRIPTION OF THE DRAWINGS

It is noted that the appended drawings illustrate only exemplaryembodiments of the invention and are, therefore, not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1 (Prior Art) is a block diagram of packet-based communicationswhere a slave clock is synchronized to a master clock over apacket-switched network.

FIG. 2 (Prior Art) is a timing diagram of packet-based methods forsynchronization involving master time-stamps and slave time-stamps.

FIG. 3A (Prior Art) is a block diagram representing a traditionalapproach to generating time-stamps.

FIG. 3B (Prior Art) is a timing diagram that reflects the time-stampgranularity error introduced in traditional approaches.

FIG. 4 is a block diagram of an embodiment for a system utilizing aslave time-stamp clock having a rate that is different from the rate ofthe master time-stamp clock.

FIG. 5 is a block diagram representing example embodiments havingdifferent clock rates for the Master and Slave time-stamp clocks.

FIG. 6 is a block diagram of an embodiment where a traditionalsyntonized time-stamp and a randomized time-stamp are used to detect asystematic granularity error.

FIG. 7A is a block diagram of an embodiment for using a spread spectrumtechnique for randomization of time-stamp counter effective values.

FIG. 7B is a block diagram of an alternate embodiment for using a spreadspectrum technique for randomization of time-stamp counter effectivevalues.

FIG. 8 is a block diagram of an embodiment for using an incrementmodulation technique for randomization of time-stamp counter effectivevalues.

DETAILED DESCRIPTION OF THE INVENTION

Systems and methods are disclosed for utilizing randomized slave(receive) time-stamp clock rates that are different from master (sender)time-stamp clock rates to reduce systematic time-stamp granularityerrors in the communication of network packets. Various features andvariations can be implemented, if desired, and related systems andmethods can be utilized, as well.

Network communication protocols utilize time-stamps to facilitate thesynchronization of network communications. As described above, manytraditional time-stamping methods use a clock rate that is the same forthe Master (source/master/server) and Slave (destination/slave/client)devices. For example, with respect to Gigabit/sec Ethernetcommunications, the MII (media independent interface) clock rate (e.g.,125 MHz for) may be used by master (sender) and slave (receive) devicesto generate time-stamps for network synchronization. However, as furtherdescribed above, this use of syntonized, equi-rate, time-stamp clocksgives rise to anomalies in the time-stamp granularity error, and thesesystematic granularity errors are difficult if not impossible to filterout.

The randomized slave time-stamps described herein reduce systematicerrors in granularity noise experienced by the network communications.More particularly, the methods and systems described herein reduce thesesystematic granularity errors by intentionally offsetting or randomizingthe Slave (receive) clock rate used to generate time-of-arrivaltime-stamps from the Master (sender) clock rate used to generatetime-of-departure time-stamps. Further, by making the rates relativelyprime with respect to each other, or otherwise randomizing the rateswith respect to each other, the spectral characteristics of thetime-stamp granularity noise can be spread out over a wideFourier-frequency range making these errors relatively easy to filterout. This improved noise spectrum permits more accurate synchronizationof network packet communications.

As described above, on the slave side, the time-stamp granularity noisecan be written as follows:

ε_(TS) ^((S))(n)=−½(ε₃(n)+ε₂(n))  (Eq. 6)

where ε₃(n) and ε₂(n) are the time-stamp granularity noise contributionsfor time-stamps T₃(n) and T₂(n), respectively. The negative sign issolely for convenience. When considering errors the more relevantmetrics are related to “power” and for that the sign is moot. Likewise,on the master side, the time-stamp granularity noise can be written asfollows:

ε_(TS) ^((M))(n)=−½(ε₄(n)+ε₁(n))  (Eq. 7)

where ε₄(n) and ε₁(n) are the time-stamp granularity noise contributionsfor time-stamps T₄(n) and T₁(n), respectively.

According to the techniques described herein, by applying arandomization technique to the Slave clock rate as compared to theMaster clock rate, the behavior of {ε₂(n)} and {ε₄(n)} granularity noisebecomes in effect a uniformly-distributed, white-noise, random processwith respect to time-of-arrival time-stamps generated by the Slavedevice. Further, the {ε₁(n)} and {ε₃(n)} granularity noise for thetime-of-departure time-stamps can be reasonably assumed also to be auniformly-distributed, white-noise, random process. As random processes,these errors can be removed with subsequent processing, such as throughthe use of filtering techniques. As such, the granularity noise causedby systematic granularity errors introduced by syntonized but offsettime-stamp clocks can be effectively reduced or removed by using therandomized slave time-stamping techniques described herein.

It is further noted that the Master (sender) device referred to hereincan be considered any desired sending, transmitting and/or sourcedevice, and the Slave (receive) device referred to herein can beconsidered any desired receiving and/or destination device. In mostnetwork communication systems, devices both send and receive networkpackets. As such, these network devices would be both Master or sendingdevices when sending or transmitting packets that include sendertime-stamps, and these devices would be Slave or receiving devices whenreceiving packets and generating receive time-stamps.

FIG. 4 is a block diagram of an embodiment 400 for a system utilizingrandomized time-of-arrival time-stamp clocks, as described herein, thathave clock rates for slave time-stamp clocks that are intentionallydifferent from the clock rates for the master time-stamp clocks. Forconvenience, the master-to-slave direction is considered with respect toFIG. 4, as the reverse from slave-to-master would be similar. Asdepicted, the Master time-stamp clock 404 has a master clock rate(f_(M)), and the Slave time-stamp clock 414 has a slave clock rate(f_(S)) that is intentionally different from the Master time-stamp clockrate (f_(S)≠f_(M)). This randomizing of clock rates allows forsystematic granularity errors to be reduced or eliminated withdownstream filtering techniques, such as low-pass filters in clockrecovery loops.

It is again noted that the master device is the sender device sending ortransmitting the initial timing packets, and the slave device is thereceive device receiving the these timing packets. The slave device willtypically then send back timing packets to the master device. It isfurther noted that any particular device that is participating innetwork communications can be a master/sender device (i.e., sending,transmitting and/or source device), a slave/receive device (i.e.,receiving and/or destination device), or can be both a master and aslave device, depending upon the direction of the timing packetcommunications occurring across the network.

Looking in more detail to embodiment 400, a master packet buffer 402provides packets to a packet transmission interface 408 where mastertime-stamps (TS_(M)) are inserted into timing packets. These packetsthat include master time-stamps (TS_(M)) are then transmitted from themaster device through network 410 to a packet reception interface 412 ata slave device. To generate the master time-stamps (TS_(M)), the mastertime-stamp clock (f_(M)) 404 provides a timing signal to the packettransmission interface 408 and the master time-stamp generator 406. Themaster time-stamp generator 406 then provides master time-stamps(TS_(M)) to the packet transmission interface 408 for inclusion inoutgoing packets. In some implementations, the time-stamp (TS_(M)) isprovided to the master packet buffer 402 and inserted into a subsequentpacket.

To generate slave time-stamps (TS_(S)), the slave time-stamp clock(f_(S)) 414 provides a timing signal to the packet reception interface412 and to the slave time-stamp generator 416. The packet receptioninterface also provides an event signal (EVENT) to the time-stampgenerator 416 to indicate the reception of a packet. The slavetime-stamp generator 416 then provides slave time-stamps (TS_(S)) to theslave packet buffer 418, where the master time-stamps (TS_(M)) arerecovered and stored along with the slave time-stamps (TS_(S)). Thesetime-stamps (TS_(M), TS_(S)) can then be used by the Slave device tosynchronize its operations to the Master clock and/or for other desiredpurposes.

Advantageously, by randomizing the slave clock frequency and thereby therate (f_(S)) for the Slave time-stamp clock 414 with respect to themaster clock frequency and thereby the rate (f_(M)) for the Mastertime-stamp clock 404, systematic granularity errors can effectively bereduced or eliminated by subsequent processing of the time-stamps, forexample, in timing recovery circuitry using low-pass filteringtechniques. It is noted that the master (sender) time-stamp generator406 and the slave (receiver) time-stamp generator 416 can include atime-stamp counter and a time-stamp register, as shown in FIG. 3A (PriorArt). And the master time-stamp clock 404 and slave time-stamp clock 414can be used to control the counting operation of the counters with thetime-stamp register being configured to store a time-stamp based uponthe occurrence of the detected event.

It is further noted that the randomizing techniques described hereinshare a common principle wherein the time-stamp clock at the slave(receiver) has a rate that is randomized with respect to and/ordifferent from, and preferably relatively prime to, the rate of thetime-stamp clock in the master (transmitter). And because the slavepossesses knowledge of the variations of its time-stamp clock, it canuse this knowledge to achieve enhanced performance Whereas the slave andmaster time-stamp may be traditionally synchronized and, therefore, thefrequency offset is very small (i.e., nearly zero parts-per-million oralmost zero in fractional frequency units), by making sure that therates of the clocks are different, the relative phase of the time-stampclock edges will change from clock-cycle to clock-cycle and thereforefrom packet to packet. Thus, because the relative phases are differentfrom clock-cycle to clock-cycle, the instantaneous phase between themaster clock and the slave clock will be different over time. Theimplication of this randomizing is that the time-stamp granularity errorwill not be constant and will have significant high-frequencycomponents. These high-frequency components can effectively be filteredout by subsequent processing, such as through the use of low-passfiltering techniques included within a clock recovery phase-locked-loop.

The randomization of the slave time-stamp clock can be illustrated withthe following examples.

Suppose the master side delivers packets that, for the purposes ofillustration, are aligned to a rising edge of the nominal transmittime-stamp clock in the Master. Assume that the nominal rate of thisclock is f_(M) and that the time-stamp clock rate in the receiver isf_(S). Also, further assume that f_(S)>f_(M). If the ratio of thetime-stamp clock frequencies (f_(S)/f_(M)) is an integer, say K, then apacket time-of-arrival will always be a fixed time offset from theclosest rising edge of the receive time-stamp clock just prior to thetime-of-arrival. This is indicated by the difference (Δ) 370 in FIG. 3B(Prior Art) for the case K=1. It is noted that the ratio can also beconsidered with respect to clock time periods of the master clock timeperiod (T_(M)) to the slave clock time period (T_(S)), such that theratio is T_(M)/T_(S).

In contrast, if the frequency ratio (f_(S)/f_(M)) is not an integer butis, for example, a rational number such as R/T, then there are exactly Rcycles of the receive time-stamp clock for every T cycles of thetransmit time-stamp clock. Depending on which edge the transmit packetis aligned with, there will be T different possible values for thedifference (Δ) 370. As recognized with respect to the randomized slavetime-stamp clocks implementations described herein, this rational numberrelationship (K=R/T) is an improvement over the case where K=1, suchthat there is only one value for the difference (Δ) 370. As describedabove, one value for the difference (Δ) 370 leads to systematicgranularity errors that cannot be detected or removed.

It is noted that the larger the value of T (e.g., R and T have fewcommon factors), the more random will be the value of the difference (Δ)370. And if the rates are relatively prime, then T is effectivelyinfinite, and the differences (Δ) 370 for a plurality of packets will beuniformly distributed over the time interval [0, Δ_(S)] whereΔ_(S)=1/f_(S) represents the period of the receive time-stamp clock. Itis noted that relatively prime refers to two integers that relate toeach other such that one is greater than 1000 after common factors areremoved.

Thus, it is desirable to randomize the Slave time-stamp clock rate withrespect to the Master time-stamp clock rate such that the ratio of theirclock rates is not an integer but is a rational number. And it isfurther desirable that this rational number be such that it takes alarge number of master time-stamp clock cycles (T) before an edge of themaster time-stamp clock aligns with an edge of the slave time-stampclock cycles (R).

Whereas a large value of T is preferable, it may be infeasible to make Textremely large (such as approaching infinity). One guideline forchoosing T is to make the natural period larger than the time-constantof the subsequent low-pass filtering function. For example, if the timeconstant of the clock recovery loop is 1000 seconds, and the nominalpacket rate is 1 second, then choosing T to be greater than 1000 (e.g.,1024) is a good compromise between implementation complexity andperformance It is noted that as T increases from T=1, which was used inprior solutions where K=R/T=1, to 1024 or higher, a continualimprovement in performance will be achieved. However, for the exampledescribed, increasing T beyond 1024 may lead to a further improvement inperformance that is not so dramatic.

FIG. 5 is a block diagram of an embodiment 500 implementing a solutionwhere the ratio of the time-stamp clock frequencies (f_(S)/f_(M)) is arational number represented by R/T. As depicted, slave time-stamp clock414 has a slave clock frequency (f_(S)) that is related to the masterclock frequency (f_(M)) of the master time-stamp clock 404 by anon-integer number. As such, the ratio of the time-stamp clockfrequencies (f_(S)/f_(M)) is a rational number. This relation can berepresented by the following equation:

(f _(S) /f _(M))=(T _(M) /T _(S))=R/T  (Eq. 8)

where R/T is preferably a rational number (i.e., R and T are integers),and where T is preferably a large number.

As one implementation, the slave time-stamp clock rate (f_(S)) can beselected to be slightly offset from the master time-stamp clock rate(f_(M)). For example, with respect to a master time-stamp clock rate of125 MHz, as describe above for Gigabit/sec Ethernet communications, aslave time-stamp clock rate of 124 MHz would be an advantageous rate tochoose. Other unrelated (e.g., non-integer related) clock rates withrespect to the master clock rate could also be selected for the slavetime-stamp clock rate (f_(S)), as desired. Further, clock rates thathave an integer relationship greater than one could also be used, suchthat the ratio of their rates (K) is an integer greater than one (e.g.,K=R/T, T=1, and R>1). This selection of unrelated and/or different clockrates for the slave time-stamp clock effectively randomizes thegranularity errors that would otherwise be systematic granularity errorsin prior solutions where slave time-stamp clocks are intentionallysyntonized with the master time-stamp clock and have the same nominalclock rate (f_(S)≈f_(M)).

In addition to selecting a fixed slave time-stamp clock rate (f_(S))that is unrelated to the master time-stamp clock rate (f_(M)), the slavetime-stamp clock rate (f_(S)) could also be varied over time torandomize the slave time-stamp clock rate (f_(S)) with respect to themaster time-stamp clock rate (f_(M)). As such, the frequency ratio(f_(S)/f_(M)) addressed above could be nominally equal to one but wouldbe time-varying about this nominal value. By using such a time-varyingclock rate (f_(S)) for the slave time-stamp clock, the slave time-stampclock rate (f_(S)) would still be different from or randomized withrespect to the master time-stamp clock rate (f_(M)) and provide theadvantages described herein. Further, if desired, the slave time-stampclock frequency can be varied over time such that it has the sameaverage frequency as the frequency for the master time-stamp clock. Forexample, the slave time-stamp clock rate (f_(S)) could be implemented byhaving a clock frequency that is syntonized with the master clock rate(f_(M)) but that also has intentional phase variations (which are alsoknown as jitter or spread spectrum clocking). In summary, a time-varyingslave frequency, and an average slave frequency matching the masterfrequency, can be represented with the following equations:

(f _(S) /f _(M))=(T _(M) /T _(S))=time-varying  (Eq. 9)

(f _(S) /f _(M))_(AVERAGE)=(T _(M) /T _(S))_(AVERAGE)=1  (Eq. 10)

In one example, a time-varying slave clock rate (f_(S)) that has anaverage nominal value equal to the master clock rate (f_(M)), or equalto a rational multiple of an average value for the master clock rate(f_(M)), could be implemented using spread spectrum techniques. In otherwords, the slave time-stamp clock can be implemented as a spreadspectrum clock, where the clock frequency is spread over a relativelywide range of frequencies instead of being concentrated at a singleclock/carrier frequency. In such an embodiment, the slave time-stampclock rate (f_(S)) can be nominally set to equal the master time-stampclock rate (f_(M)), and the slave time-stamp clock rate (f_(S)) can thenbe intentionally dithered using spread spectrum techniques (e.g.,applying a pseudo-random spreading code) to randomize the slavetime-stamp clock with respect to master time-stamp clock, therebyallowing for systematic granularity errors to be reduced or eliminated.

It is noted that other randomizing techniques could also be applied tothe slave time-stamp clock, as desired, to randomize the slavetime-stamp clock rate (f_(S)) with respect to master time-stamp clockrate (f_(M)) so that systematic granularity errors can be reduced oreliminated. One particular example of a randomizing technique that canbe applied is delta-sigma modulation, especially higher orderdelta-sigma modulation, which produces randomness having theadvantageous property that the spectral components of the noise areshaped to be predominantly high frequency and which further simplifiesthe aforementioned low-pass filtering process.

FIG. 6 is a block diagram of an embodiment 600 for a implementationwhere two time-stamps are used to detect a systematic granularity error.As depicted, a master time-stamp clock (f_(M)) 404 is provided to amaster time-stamp generator 406 to produce a master time-stamp (TS_(M))602. As described herein, a slave time-stamp (TS_(S-S)) 614 is generatedby slave time-stamp generator 610 using a randomized slave time-stampclock 608 operating at a rate (f_(S)) that is different from the rate(f_(M)) of the master time-stamp clock 404. In addition, a slavetime-stamp (TS_(S-M)) is also generated by the slave time-stampgenerator 610 using a standard slave time-stamp clock 606 operating at arate (f_(M)) implying that is the same rate, and possibly syntonized,with the master time-stamp clock 402.

By taking time-stamps using both a standard syntonized slave time-stampclock 606 and a randomized slave time-stamp clock 608 at the same time,a determination can be made of the existence and magnitude of anysystematic granularity errors being experienced by the system. In otherwords, while time-stamps struck using the syntonized slave time-stampclocks, according to prior solutions, lead to systematic granularityerrors, time-stamps struck using the randomized slave time-stamp clocksdescribed herein provide time-stamps from which such granularity errorscan be removed. Thus, further time-stamp processing, such as statisticalanalysis of the two timestamps (e.g., syntonized, randomized), can beused to indicate that the systematic granularity error is present and toindicate the magnitude of this error.

FIGS. 7A, 7B, and 8 provide additional example embodiments that utilizespread spectrum and/or delta-sigma modulation techniques to effectivelyrandomize the slave time-stamp clock. For these embodiments, atime-stamp generator is configured to generate receive time-stampsassociated with the received network packets using a receive time-stampclock signal having a receive time-stamp clock rate, and an effectiverate for the time-stamp clock is varied over time around a mean value torandomize timing for the receive time-stamps with respect to a sendertime-stamp clock rate.

For the embodiment 700 of FIG. 7A and for the embodiment 750 for FIG.7B, the time-stamp clock generator includes a numerically controlledoscillator configured to have a clock signal and a conversion value asinputs and configured to output a signal that is used to provide thereceive time-stamp clock signal. The conversion value is furtherconfigured to be varied over-time around a mean value to cause theeffective rate for the time-stamp clock to vary over time around a meanvalue.

For the embodiment 800 of FIG. 8, the time-stamp generator includes anadder configured to have a conversion value and a current time-stampvalue as inputs and includes a register coupled to receive an outputfrom the adder. The register is further configured to be clocked by thereceive time-stamp clock signal to store the output from the adder, andthe conversion value is configured to be varied over-time around a meanvalue to cause the effective rate for the time-stamp clock to vary overtime around a mean value. These embodiments will now be described inmore detail.

FIG. 7A is a block diagram of an embodiment 700 for using a spreadspectrum technique for randomization of time-stamp counter effectivevalues. It is noted that the embodiment 700 is directed to theslave-side time-stamping device for convenience. An equivalent approachcan be applied to the master-side time-stamping device.

For the embodiment 700 depicted, the current time-stamp value 745 isobtained as the output of the time-stamp counter 740. The currenttime-stamp value 745 can be loaded into a time-stamp register (notdepicted) as determined by the occurrence of the event beingtime-stamped as depicted in FIG. 3A (Prior Art). The time-stamp counter740 is clocked by the time-stamp counter clock (f_(S)) 720. For theembodiment 700 depicted, this time-stamp clock (f_(S)) 720 is generatedusing a numerically controlled oscillator (NCO) 730 that in turn isdriven by a high-speed system clock (f_(HS)) 710. The NCO 730 operatesto generate the time-stamp counter clock (f_(S)) 720, depending upon thevalue of a conversion parameter (R_(eff)) 754. The relationship of thetime-stamp counter clock (f_(S)) 720 to the high-speed system clock(f_(HS)) 710 and the conversion parameter (R_(eff)) 754 can berepresented by the following equation: f_(S)=R_(eff)f_(HS)G, where “G”represents an implicit frequency scaling factor of the NCO 730.

The conversion parameter (R_(eff)) 754 can be implemented as atime-variable value that varies around a mean or nominal value in orderto randomize the time-stamp clock (f_(S)) 720. For the embodiment 700depicted, the conversion parameter (R_(eff)) 754 is generated by addinga variable term X(t) 760 to a nominal value (R) 750 using summer 752.The variable term X(t) 760 can be implemented, for example, as azero-mean noise sequence. Because the variable term X(t) 760 iszero-mean, on the average, the following equation will be satisfied:f_(S)≈Rf_(HS)G. In operation, the significant edges of the time-stampcounter clock (f_(S)) 720 will vary around the nominal (mean) positionthereby introducing the desired spread-spectrum behavior.

The additive noise term X(t) 760 can be generated, for example, by usinga pseudo-random number generator. The additive noise X(t) 760 can alsobe constrained such that its maximum absolute value is small, forexample, less than about 1% of the nominal value (R) 750, if desired.The additive noise X(t) 760 can also be configured to change slowlyrelative to the rate of the time-stamp clock (f_(S)) 720. For example,the rate of the time-stamp clock (f_(S)) 720 can be configured to be onthe order of 125 MHz, and the rate of change of the variable term X(t)760 can be configured to be of the order of the packet-transmission ratewhich is typically of the order of 1 Hz. Other variations could beimplemented, as desired.

FIG. 7B is a block diagram of an embodiment 750 which provides analternate approach to generate the time-stamp counter clock 720 of FIG.7A. For the embodiment 750, a phase locked loop arrangement is depictedwherein the time-stamp counter clock signal 720 is generated by avoltage controlled oscillator (VCO) 782 and the control input signal forthe VCO 782 is generated by filtering the phase difference between areference clock (REF CLK) 770 and a feedback clock signal 771. Thefeedback clock signal 771 is generated from the time-stamp counter clocksignal 720 with a numerically controlled oscillator (NCO) 790. The NCO790 has the time-stamp clock signal 720 and a conversion value (R_(eff))755 as inputs. The conversion value (R_(eff)) 755 is further configuredto be varied over-time around a mean value to cause the effective ratefor the time-stamp clock 720 to vary over time around a mean value.

In operation, the embodiment 750 depicted in FIG. 7B operates similar toembodiment 700 of FIG. 7A except that an alternative method is used togenerate the time-stamp counter clock (f_(S)) 720. As depicted, thetime-stamp clock (f_(S)) 720 is generated using VCO 782, and VCO 782 isvaried according to input control voltage applied to VCO 782. The outputof the phase-frequency-detector (PFD) 775 is filtered by loop filter780, and the output of the loop filter 780 provides the input controlvoltage to VCO 782. The PFD 775 determines the difference between thereference clock 770 and the feedback signal 771, and outputs a signalindicative of this difference. The feedback signal 771 is based upon theoutput of NCO 790, as adjusted by the pre-scale block (÷M) 772, whichprovides a divide-by-M operation. The NCO 790 receives the time-stampcounter clock 720 and the conversion parameter (R_(eff)) 755 andprovides the feedback signal to the pre-scaling block (÷M) 772. Thus,the NCO 790 is driven by the time-stamp counter clock 720 as modified bythe conversion parameter (R_(eff)) 755 to produce this feedback outputsignal. The relationship of the time-stamp counter clock (f_(S)) 720 tothe reference clock (f_(REF)) 770 and the conversion parameter (R_(eff))755 can be represented by the following equation:f_(S)=R_(eff)f_(REF)GM, where G represents the implicit frequencyscaling factor of the NCO 790, where f_(REF) represents the rate of thereference clock 770, and where M represents the pre-scaling parameter of772. It is noted that the pre-scale block (÷M) 772 could be removed, ifdesired.

The conversion parameter (R_(eff)) 755 can be implemented as atime-variable value that varies around a mean or nominal value in orderto randomize the time-stamp clock (f_(S)) 720. For the embodiment 750depicted, the conversion parameter (R_(eff)) 755 is generated by addinga variable term X(t) 761 to a nominal value (R) 751 using summer 753.The variable term X(t) 761 can be implemented, for example, as azero-mean noise sequence. Because X(t) 761 is zero-mean, on the averagethe following equation will be satisfied: f_(S)≈Rf_(REF)GM. Inoperation, the significant edges of the time-stamp counter clock (f_(S))720 will vary around the nominal (mean) position thereby introducing thedesired spread-spectrum behavior.

The additive noise term X(t) 761 can be generated, for example, by usinga pseudo-random number generator. The additive noise X(t) 761 can alsobe constrained such that its maximum absolute value is small, forexample, less than about 1% of the nominal value (R) 751, if desired.The additive noise X(t) 761 can also be configured to change slowlyrelative to the rate of the time-stamp clock (f_(S)) 720. For example,the rate of the time-stamp clock (f_(S)) 720 can be configured to be onthe order of 125 MHz, and the rate of change of the variable term X(t)761 can be configured to be of the order of the packet-transmission ratewhich is typically of the order of 1 Hz. Other variations could beimplemented, as desired.

FIG. 8 is an block diagram of an embodiment 800 for using an incrementmodulation technique for randomization of time-stamp counter effectivevalues. It is noted that the embodiment 800 is directed to theslave-side time-stamping device for convenience. An equivalent approachcan be applied to the master-side time-stamping device.

For the embodiment 800 depicted, the current time-stamp value 845 isobtained as the output of an accumulator 830, and the accumulator 830effectively operates as a time-stamp counter. The current time-stampvalue 845 can be loaded into a time-stamp register (not depicted) asdetermined by the occurrence of the event being time-stamped as depictedin FIG. 3A (Prior Art). For the embodiment 800 depicted, the accumulator830, which implements the time-stamp counter, is clocked by thehigh-speed system clock (f_(HS)) 810. The accumulator 830 includes anadder 832 that provides an output to a register 834, which is clocked bythe high-speed system clock (f_(HS)) 810. The output of the register 834is then provided back to the adder 832, which also receives a conversionparameter (R_(eff)) 854. This operation of the accumulator 830 emulatesa counter driven by a clock of frequency (f_(S)) as represented by thefollowing equation: f_(S)=R_(eff)f_(HS).

The conversion parameter (R_(eff)) 754 can be implemented as atime-variable value that varies around a mean or nominal value in orderto randomize the value added by adder 832 to the output of register 834and thereby the emulated time-stamp clock (f_(S)). For the embodiment800 depicted, the conversion parameter (R_(eff)) 854 is generated byadding a variable term X(t) 860 to a nominal value (R) 850 using summer852. The variable term X(t) 860 can be implemented, for example, as azero-mean noise sequence. Because the variable term X(t) 860 iszero-mean, on the average, the following equation will be satisfied:f_(S)≈Rf_(HS). Equivalently, the time increment will correspond to oneperiod of the emulated time-stamp clock (f_(S)). In operation, theincrements of the time-stamp counter provided by accumulator 830 willvary around the nominal (mean) value, thereby introducing a desiredfractional increment.

The additive noise term X(t) 860 can be generated, for example, by usinga pseudo-random number generator. Further the additive noise term X(t)can be constrained, if desired, such that its maximum absolute value issmall, for example, less than about 1% of the nominal value (R) 850, ifdesired. The additive noise X(t) can also be configured to change slowlyrelative to the rate of the emulated time-stamp clock (f_(S)). Forexample, the rate of the emulated time-stamp clock (f_(S)) can beconfigured to be of the order of 125 MHz, and the rate of change of X(t)can be configured to be of the order of the packet-transmission ratewhich is typically of the order of 1 Hz. Other variations could beimplemented, as desired.

It is noted that the variable increment term X(t) 860 is effectively theequivalent of a fraction. Thus, if the nominal value (R) 850 isconsidered to be an integer, then the effective value (R_(eff)) 854output by the summer 852 will be a fractional value between R and (R+1)on the average. Assuming this fractional value is represented by “η,”the effective value (R_(EFF)) 854 can be viewed as a sequence that willtake on values 0 and 1 such that over a period of time the average valueis η. This result can be achieved, if desired, using adelta-sigma-modulator to convert the fraction η into apulse-density-modulated stream. This pulse-density-modulate stream canthen be applied as the variable term X(t) 860.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description. Itwill be recognized, therefore, that the present invention is not limitedby these example arrangements. Accordingly, this description is to beconstrued as illustrative only and is for the purpose of teaching thoseskilled in the art the manner of carrying out the invention. It is to beunderstood that the forms of the invention herein shown and describedare to be taken as the presently preferred embodiments. Various changesmay be made in the implementations and architectures. For example,equivalent elements may be substituted for those illustrated anddescribed herein, and certain features of the invention may be utilizedindependently of the use of other features, all as would be apparent toone skilled in the art after having the benefit of this description ofthe invention.

What is claimed is:
 1. A receiving device for network communications,comprising: a packet reception interface configured to receive networkpackets from a sending device; a time-stamp generator configured togenerate receive time-stamps associated with received network packetsusing a receive time-stamp clock signal having a receive time-stampclock rate, wherein an effective rate for the time-stamp clock variesover time around a mean value to randomize timing for the receivetime-stamps with respect to a sender time-stamp clock rate for a sendertime-stamp clock signal used to generate sender time-stamps; and atime-stamp clock generator configured to generate the receive time-stampclock signal having the receive time-stamp clock rate.
 2. The receivingdevice of claim 1, wherein the time-stamp clock generator comprises anumerically controlled oscillator configured to have a clock signal anda conversion value as inputs and configured to output a signal used toprovide the receive time-stamp clock signal, the conversion value beingconfigured to be varied over-time around a mean value to cause theeffective rate for the receive time-stamp clock signal to vary over timearound a mean value.
 3. The receiving device of claim 2, wherein theconversion value is configured to be varied over-time using a variableinput value summed with a nominal input value.
 4. The receiving deviceof claim 3, wherein the variable input value is configured to be basedupon a pseudo-random number to apply a spread spectrum technique to theconversion value.
 5. The receiving device of claim 1, wherein thetime-stamp generator comprises an adder configured to have a conversionvalue and a current time-stamp value as inputs and a register coupled toreceive an output from the adder, the register being configured to beclocked by the receive time-stamp clock signal to store the output fromthe adder, and the conversion value being configured to be variedover-time around a mean value to cause the effective rate for thereceive time-stamp clock signal to vary over time around a mean value.6. The receiving device of claim 5, wherein the conversion value isconfigured to be varied over-time using a variable input value summedwith a nominal input value.
 7. The receiving device of claim 6, whereinthe variable input value is configured to be based upon a pseudo-randomnumber to apply a spread spectrum technique to the conversion value. 8.The receiving device of claim 6, wherein the variable input value isconfigured to be based upon a pulse-density modulated stream to apply adelta-sigma modulation technique to the conversion value.
 9. A methodfor network communications, comprising: receiving at a receiving devicenetwork packets from a sending device, the network packets includingsender time-stamps generated at the sending device using a sendertime-stamp clock signal having a sender time-stamp clock rate;generating receive time-stamps associated with received network packetsusing a receive time-stamp clock signal having an effective rate thatvaries over time around a mean value to randomize timing for the receivetime-stamps with respect to the sender time-stamp clock rate; andutilizing the sender time-stamps and the receive time-stamps for timealignment within the receiving device.
 10. The method of claim 9,further comprising generating the receive time-stamp clock signal usinga numerically controlled oscillator having a clock signal and aconversion value as inputs, and varying the conversion value over-timearound a mean value to cause the effective rate for the receivetime-stamp clock signal to vary over time around a mean value.
 11. Themethod of claim 10, further comprising varying the conversion valueover-time using a variable input value summed with a nominal inputvalue.
 12. The method of claim 10, wherein the conversion value isvaried using a pseudo-random number to apply a spread spectrum techniqueto the conversion value.
 13. The method of claim 9, wherein thegenerating step comprises adding a conversion value to a currenttime-stamp value to generate the receive time-stamps, storing thereceive time-stamps based upon the receive time-stamp clock signal, andvarying the conversion value over-time around a mean value to cause theeffective rate for the receive time-stamp clock signal to vary over timearound a mean value.
 14. The method of claim 13, further comprisingvarying the conversion value over-time using a variable input valuesummed with a nominal input value.
 15. The method of claim 13, whereinthe conversion value is varied using a pseudo-random number to apply aspread spectrum technique to the conversion value.
 16. The method ofclaim 13, wherein the conversion value is varied using a pulse-densitymodulated stream to apply a delta-sigma modulation technique to theconversion value.